1. Field of the Invention
The present invention relates to a method and an apparatus for verifying influence of chippings, which are caused in a power supply layer, on electromagnetic wave radiation in a circuit board in which a wiring layer between components to be mounted faces the power supply layer through an insulating layer intervening between the wiring layer and the power supply layer.
2. Description of Related Art
A printed circuit board for use in recent electronic equipment is provided with conductive layers, to each of which a plurality of power supply voltages are applied, and conductive layers to be separated according to power supply voltages from each other and to be held at a common potential. In the specification, these conductive layers will be referred to as power layers. In the printed circuit board, an insulating layer separates the power layer from the wiring layer. For example, a uniform-thickness insulating layer is disposed between a signal line and the power layer in a microstrip-line structure.
The power layer to which a supply voltage is applied, (hereunder referred to as the supply voltage plate), is under layout restrictions that a plurality of power layers are efficiently disposed, and that the power layer should be disposed in such a way as to avoid via-forming regions. Thus, chippings, such as “gaps”, “slots” and “slits” are often formed in the power layer. Incidentally, the “chippings” are defined herein as chipped-off parts of a power layer, which correspond to one signal line and interrupt regions each containing a wiring layer and a power layer that are opposed to each other. Therefore, a part, in which a power layer is partly absent, at a place, in which no signal line faces the power line, does not belong to the category of the chipping. The “gaps” are defined herein as chippings in a power layer, which are affected mainly by the length in the wiring direction of the signal line. Usually, the clearance between two power layers is referred to as the “gap”. In a case where a signal line intersects with a part sufficiently away from both ends of an elongated opening portion formed in one power layer, this opening portion is also regarded for the signal line as a “gap” Conversely, in a case where the end portions of an opening portion formed in a power layer are disposed closely to both sides in the direction of width of a signal line, respectively, and that both the end portions thereof affect the signal line, this opening portion is referred to as a “slot”. In a case where only one of end portions of an opening portion formed therein is close to a signal line and affects signal line, this opening portion is referred to as a “slit”.
Implementation of wiring design for separating ground plates of an analog circuit and a digital circuit from each other or avoiding the power layers to be disposed in via-forming regions sometimes results in formation of gaps at initially unexpected parts owing to requirements concerning power layers (hereunder referred to as “common potential plates”) held at common potential, which are, for example, that the ground plates of an analog circuit and a digital circuit are separated from each other and that the power layers should be disposed in such a manner as to avoid via-forming regions.
It is known that when a signal line is present in such a way as to intersect with a gap, the intensity of electromagnetic wave radiation from the signal line increases as a result of a fact that a power supply plate (that is, a supply voltage plate or a common potential plate) opposed to the signal line is missing at an intersection part. This is caused by a fact that a return path of current flowing through the signal line becomes away therefrom.
A method of connecting the plates, which are spaced with a gap, to each other by an electric conductor so as to reduce the influence of chippings (mainly including a gap) of the power supply plate opposed to the signal line is known as a measure for restraining unnecessary electromagnetic wave radiation.
However, even when this measure is used, it is frequent that the plates cannot be connected to each other by an electric conductor, for instance, in a case that different voltages are actually applied to between the power supply plates. Although electromagnetic wave radiation is reduced when dimensions of the gap are small, it is not easily determined what dimensions of the gap are sufficient to reduce the electromagnetic wave radiation to an extent that resultant electromagnetic wave radiation is small enough to be ignored. It takes much time and effort to find a gap by visual inspection in a miniaturized high-density mounting printed circuit board. It is highly likely to overlook the gap.
Japanese Patent Application Publication Laid-Open No. 2001-331539 proposes a method of using simple computational expressions to verify whether or not a signal line transversing a target opening portion (that is, a slot or a slit) of a common potential plate, is present, without substantially changing a related art design process and without considerably increasing a design cost. The above-mentioned official gazette discloses a method of simply checking the dimensions of the opening portion in a case where a signal line transversing the opening portion.
Concretely, according to the method described in the official gazette, a dimension (indicated by “Slot”) of an opening portion 101b of a power supply plate 100b illustrated in FIG. 12B, and a distance (designated by “Slit”) between an opening portion 101a of a power supply plate 100a and the center of the signal line illustrated in FIG. 12A are calculated by using predetermined computational expressions. Thus, it is verified whether or not an opening portion is provided in the target power supply plate. When the opening portion 101b is provided therein, the dimension of the opening portion 101b is outputted and displayed. Moreover, it is verified whether or not a signal line is present in the vicinity of the target opening portion 101a of the power supply plate. When a signal line is present, it is judged whether or not the distance between the opening portion 101a and the signal line is appropriate. According to a result of the judgment, an instruction for countermeasures is displayed. This official gazette describes that consequently, a design engineer can easily check what dimensions of the target opening portion 101b of the power supply plate, is appropriate, and what distance between the opening portion 101a and the signal line is appropriate, without substantially changing a related art design process and without needing additional design cost.
However, the technique described in the aforementioned official gazette has a drawback that in a case where many parts of the circuit board, which are referred to as the slots (see FIG. 12B) and the slits (see FIG. 12A), are to be adjusted, the degree of influence of each of the parts, which are to be adjusted, on occurrences of an electromagnetic wave radiation cannot be determined, and priorities for adjusting each of the parts, according to which one of the parts is determined so that the adjustment is started therefrom, cannot easily be known. Thus, there is possibility that the parts each generating a large amount of electromagnetic wave radiation cannot be completely adjusted within a circuit board designing period when using a verifying method and a verifying system described in the aforementioned official gazette. In this case, electromagnetic wave radiation from the entire printed circuit board cannot be effectively suppressed.
In a case where one of a plurality of power layers completely separated from each other in the same plane is set to be a target plate to be verified, the technique described in the aforementioned official gazette has another drawback in that wirings (signal lines) transversing above both the separated plates cannot be recognized by the verifying system as a part to be adjusted. This is because the verifying method and system described in the aforementioned official gazette limit the opening portions to the slot (shown in FIG. 12B) and the slit (shown in FIG. 12A) and do not have a function of detecting a gap in a case where the power supply plates are completely separated from one another. Therefore, in the case of using the verifying method and system described in the aforementioned official gazette, gap parts each having a large amount of electromagnetic wave radiation may be overlooked. Consequently, the verifying method and system described in the aforementioned official gazette have a disadvantage that an amount of electromagnetic wave radiation generated from the entire printed circuit board cannot effectively be suppressed.
It is known that a cable connected to a connector serves as an antenna and radiates electromagnetic waves in a case of a circuit board on which the connector to which the cable is to be connected is employed as a component to be mounted.
However, the method and system described in the aforementioned official gazette have still another drawback in that in a case of providing a plurality of connectors on the circuit board, the system cannot obtain information indicating which of the connectors should be selected as the one corresponding to a part to which countermeasures for suppressing electromagnetic wave radiation are applied, and that thus, a complete verification of the circuit board design cannot be performed.